This invention relates generally to electrical conductors, and more particularly to a method of fabricating a compressed gas insulated cable.
Compressed gas insulated transmission lines are being used in an ever increasing scale in recent years due to the desirability of increasing safety, problems in acquiring right-of-way for overhead lines, and higher power loads required by growing metropolitan areas and growing demands for electrical energy. Compressed gas insulated transmission lines typically comprise a hollow sheath, a conductor in the sheath, a plurality of solid insulating spacers which support the conductor, and a compressed gas such as sulfur hexaflouride or the like in the sheath to insulate the conductor from the sheath. The typical assembly has been fabricated from relatively short sections of hollow cylindrical ducts or tubes in which the conductor and insulators are inserted. This assembly is usually completed in the factory, and the sections are welded or otherwise secured together in the field to form the transmission line. Gas barriers are provided at intervals along the length of the assembly, and, after evacuation of the line, an insulating gas is forced into the sheath under pressure. It is also known to provide a particle trap in compressed gas insulating transmission lines as is disclosed in the patent to Trump, U.S. Pat. No. 3,515,939. The particle trap of Trump is used to precipitate out of the insulting gas, particles of foreign matter which could adversely affect the breakdown voltage of the dielectric gas.
Problems have arisen, however, in the use of such compressed gas insulated cables. Two or more parts, the sheath and conductor, must be thoroughly cleaned separately and then assembled into the final units without introducing even the slightest amount of contamination. The clearance necessary to get the several parts together necessitates the use of folded or wedged type joints between the several parts, or purposely leaving the parts loose on plastic pads. These methods require several sequential operations over a period of time during which contamination can be produced or enter and the use of expensive tubing with special mounting provisions or complicated mounting rings which fit inside the sheath tubing.
One sheath which has been designed to overcome these problems is illustrated in the U.S. patent to Fox et al, U.S. Pat. No. 3,864,507. In the Fox et al patent, the outer sheath is constructed from sheath sectors which mate together to form the outer cylindrical sheath. However, this sheath is not entirely satisfactory, as the assembly in the field of the gas insulated cable is not very efficient, and alignment and assembly problems may occur.